Spots on the surface of Betelgeuse -- Results from new 3D stellar convection models

The observed irregular brightness fluctuations of the well-known red supergiant Betelgeuse (alpha Ori, M2 Iab) have been attributed by M. Schwarzschild (1975) to the changing granulation pattern formed by only a few giant convection cells covering the surface of this giant star. The surface structure revealed by modern interferometric methods appears to be generally consistent with the explanation as large-scale granular intensity fluctuations. The interferometric data can be modeled equally well by assuming the presence of a few (up to 3) unresolved hot or cool spots on a limb-darkened disk. In an effort to improve our theoretical understanding of the Betelgeuse phenomena, we have applied a new radiation hydrodynamics code (CO⁵BOLD) to the problem of global convection in giant stars. For this purpose, the "local box" setup usually employed for the simulation of solar-type surface convection cannot be used. Rather, we have chosen a radically different approach: the whole star is enclosed in a cube ("star-in-a-box" setup). The properties of the stellar model are defined by the prescribed gravitational central potential and by a special inner boundary condition which replaces the unresolved core, including the source of nuclear energy production. We present current results obtained from this novel generation of 3D stellar convection simulations, proceeding from a toy model ("Mini-Sun") towards the numerically more demanding supergiant regime. We discuss the basic observational properties of Betelgeuse in the light of our best model obtained so far (T_eff = 3300 K, log g = -0.4). Finally, we describe a first attempt to investigate the interaction of the global convective flows with magnetic fields based on the kinematic approximation.